In recent years, a mobile terminal is explosively widespread. A radio circuit thereof is required to be miniaturized. A radio receiving apparatus has two main types: one employing a homodyne (direct conversion) system and the other employing a heterodyne system. The heterodyne system is widely used for a mobile terminal, since the heterodyne system does not have problems of DC offset, 1/f noise, AM suppression, and the like, each of which is major interference in the homodyne system. On the other hand, the heterodyne system has a problem of interference caused by an image frequency signal.
FIG. 11 is a diagram showing a mechanism of interference caused by an image frequency signal. As shown in FIG. 11, the image frequency signal is present on the opposite side of a desired signal across a local oscillation signal LO. Therefore, when the image frequency signal and the desired signal are inputted to a mixer 900 and mixed with the local oscillation signal LO, the image frequency signal and the desired signal overlap each other in an intermediate frequency (IF) band. Accordingly, the image frequency signal interferes with demodulation of the desired signal. In order to reject the above-described image frequency signal, a steep RF filter may be simply provided so as to input only the desired signal to the mixer 900. However, the above-described RF filter increases in size, and therefore is inappropriate for a radio receiving apparatus which is required to be miniaturized.
In response thereto, a radio receiving apparatus employing the Hartley method is conventionally proposed for suppressing the above-described image frequency signal, using two local oscillation signals in phases 90 degrees different from each other. FIG. 12A is a block diagram showing a functional structure of the radio receiving apparatus employing the Hartley method. In FIG. 12A, the radio receiving apparatus includes a first mixer 901, a second mixer 902, a first phase shifter 903, and a second phase shifter 904.
The first phase shifter 903 rotates the phase of a local oscillation signal LO and divides the local oscillation signal LO into a local oscillation signal in the phase rotated by 0 degrees and a local oscillation signal in the phase rotated by 90 degrees, so as to input the respective local oscillation signals to the first mixer 901 and the second mixer 902. The first mixer 901 down-converts, by the local oscillation signal, an RF signal including a desired signal and an image frequency signal, so as to output the down-converted signals. FIG. 12B is a block diagram showing a phase relationship between the output signals from the first mixer 901 of FIG. 12A. As shown in FIG. 12B, when outputted from the first mixer 901, the desired signal and the image frequency signal are in phase.
The second mixer 902 down-converts the RF signal by the local oscillation signal in the phase rotated by 90 degrees, so as to output the down-converted signals. FIG. 12C is a block diagram showing a phase relationship between the output signals from the second mixer 902 of FIG. 12A. As shown in FIG. 12C, in the second mixer 902, the desired signal and the image frequency signal are in antiphase.
The second phase shifter 904 rotates the phases of the signals outputted from the first mixer 901 by 0 degrees and rotates the phases of the signals outputted from the second mixer 902 by minus 90 degrees, so as to combine the signals. FIG. 12D is a diagram showing phase relationships among the output signals from the second phase shifter 904 of FIG. 12A. As shown in FIG. 12D, the two image frequency signals have the same amplitude and are in antiphase, while the two desired signals have the same amplitude and are in phase. Thus, the image frequency signals are suppressed.
Ideally, the image frequency signals are rejected by the above-described image rejection of the Hartley method. In practice, however, the image frequency signals cannot be completely rejected, due to variations in elements used for the radio receiving apparatus. Therefore, a variety of radio receiving apparatuses are proposed for improving the amount of image suppression by making compensation for variations.
FIG. 13 is a block diagram showing a functional structure of a receiving circuit disclosed in Patent Document 1. In the receiving circuit disclosed in Patent Document 1, first, two streams of IF signals down-converted to an IF frequency by local oscillation signals in phases 90 degrees different from each other are generated. A circuit 906 extracts one of the IF signals so as to detect both the signals by the extracted signal. The circuit 906 detects a phase difference between the examined two signals, and adjusts a variable phase shifter 905 such that the phase difference is 90 degrees. As a result, compensation is made for phase variations, and thus it is possible to realize a receiving circuit capable of high image suppression.
FIG. 14 is a block diagram showing a functional structure of a receiving circuit disclosed in Patent Document 2. In the receiving circuit disclosed in Patent Document 2, two streams of IF signals down-converted to an IF frequency by local oscillation signals in phases 90 degrees different from each other are generated. The phase of one of the two streams of IF signals is rotated by another 90 degrees, so as to calculate the sum of and the difference between the two streams of IF signals. Further, the electric power of the difference signal is detected in a power detection circuit 907a and the electric power of the sum signal is detected in a power detection circuit 907b. The electric power of the difference signal and the electric power of the sum signal are compared to each other, so as to obtain the difference therebetween, and then a switch is flipped to the power detection circuit of the signal having the greater electric power. Then, a circuit 908 adjusts the gain of an IF amplifier so as to minimize the electric power of the power detection circuit to which the switch is flipped. As a result, compensation is made for the gain/loss of elements, i.e., amplitude variations, and thus it is possible to realize a receiving circuit capable of high image suppression.
FIG. 15 is a block diagram showing a functional structure of a receiving circuit disclosed in Patent Document 3, Patent Document 4, and Patent Document 5. In the receiving circuit shown in FIG. 15, two streams of IF signals down-converted to an IF frequency by local oscillation signals in phases 90 degrees different from each other are generated. Then, only when signals at an image frequency are stronger than signals at a desired frequency, a circuit 911 generates signals obtained by suppressing the signals at the desired frequency from a portion of the two IF signals. In other words, the circuit 911 generates signal components at the image frequency. Further, a circuit 910 adjusts the levels of the original two IF signals, and a circuit 909 subtracts the signal components at the image frequency therefrom. The levels are adjusted in attenuators ATT. An image interference canceller adjusts the amount of the attenuation so as to minimize a bit error rate (BER: Bit Error Rate) of demodulating the IF signals obtained after the subtraction. As a result, compensation is made for both phase variations and amplitude variations, and thus it is possible to realize a receiving circuit capable of high image suppression.    Patent Document 1: Japanese Laid-Open Patent Publication No. 8-125447    Patent Document 2: Japanese Laid-Open Patent Publication No. 8-130416    Patent Document 3: Japanese Laid-Open Patent Publication No. 2002-246847    Patent Document 4: Japanese Laid-Open Patent Publication No. 2003-309612    Patent Document 5: Japanese Laid-Open Patent Publication No. 2004-72532